BAG3 and BAG6 differentially affect the dynamics of stress granules by targeting distinct subsets of defective polypeptides released from ribosomes

Antagonism Between DUX4 and DUX4c Highlights a Pathomechanism Operating Through β-Catenin in Facioscapulohumeral Muscular Dystrophy

Aberrant expression of the transcription factor DUX4 from D4Z4 macrosatellite repeats on chromosome 4q35, and its transcriptome, associate with pathogenesis in facioscapulohumeral muscular dystrophy (FSHD). Forced DUX4 expression halts skeletal muscle cell proliferation and induces cell death. DUX4 binds DNA via two homeodomains that are identical in sequence to those of DUX4c (DUX4L9): a closely related transcriptional regulator encoded by a single, inverted, mutated D4Z4 unit located centromeric to the D4Z4 macrosatellite array on chromosome 4. However, the function and contribution of DUX4c to FSHD pathogenesis are unclear. To explore interplay between DUX4, DUX4c, and the DUX4-induced phenotype, we investigated whether DUX4c interferes with DUX4 function in human myogenesis. Constitutive expression of DUX4c rescued the DUX4-induced inhibition of proliferation and reduced cell death in human myoblasts. Functionally, DUX4 promotes nuclear translocation of β-CATENIN and increases canonical WNT signalling. Concomitant constitutive expression of DUX4c prevents β-CATENIN nuclear accumulation and the downstream transcriptional program. DUX4 reduces endogenous DUX4c levels, whereas constitutive expression of DUX4c robustly suppresses expression of DUX4 target genes, suggesting molecular antagonism. In line, DUX4 expression in FSHD myoblasts correlates with reduced DUX4c levels. Addressing the mechanism, we identified a subset of genes involved in the WNT/β-CATENIN pathway that are differentially regulated between DUX4 and DUX4c, whose expression pattern can separate muscle biopsies from severely affected FSHD patients from healthy. Finally, blockade of WNT/β-CATENIN signalling rescues viability of FSHD myoblasts. Together, our study highlights an antagonistic interplay whereby DUX4 alters cell viability via β-CATENIN signalling and DUX4c counteracts aspects of DUX4-mediated toxicity in human muscle cells, potentially acting as a gene modifier for FSHD severity. Importantly, direct DUX4 regulation of the WNT/β-CATENIN pathway informs future therapeutic interventions to ameliorate FSHD pathology.

Involvement of muscle satellite cell dysfunction in neuromuscular disorders: Expanding the portfolio of satellite cell-opathies

Neuromuscular disorders are a heterogeneous group of acquired or hereditary conditions that affect striated muscle function. The resulting decrease in muscle strength and motility irreversibly impacts quality of life. In addition to directly affecting skeletal muscle, pathogenesis can also arise from dysfunctional crosstalk between nerves and muscles, and may include cardiac impairment. Muscular weakness is often progressive and paralleled by continuous decline in the ability of skeletal muscle to functionally adapt and regenerate. Normally, the skeletal muscle resident stem cells, named satellite cells, ensure tissue homeostasis by providing myoblasts for growth, maintenance, repair and regeneration. We recently defined 'Satellite Cell-opathies' as those inherited neuromuscular conditions presenting satellite cell dysfunction in muscular dystrophies and myopathies (doi:10.1016/j.yexcr.2021.112906). Here, we expand the portfolio of Satellite Cell-opathies by evaluating the potential impairment of satellite cell function across all 16 categories of neuromuscular disorders, including those with mainly neurogenic and cardiac involvement. We explore the expression dynamics of myopathogenes, genes whose mutation leads to skeletal muscle pathogenesis, using transcriptomic analysis. This revealed that 45% of myopathogenes are differentially expressed during early satellite cell activation (0 - 5 hours). Of these 271 myopathogenes, 83 respond to Pax7, a master regulator of satellite cells. Our analysis suggests possible perturbation of satellite cell function in many neuromuscular disorders across all categories, including those where skeletal muscle pathology is not predominant. This characterisation further aids understanding of pathomechanisms and informs on development of prognostic and diagnostic tools, and ultimately, new therapeutics.

Regulation of Fibroblast Activation Protein-α Expression: Focus on Intracellular Protein Interactions

The prolyl-specific peptidase fibroblast activation protein-α (FAP-α) is expressed at very low or undetectable levels in nondiseased human tissues but is selectively induced in activated (myo)fibroblasts at sites of tissue remodeling in fibrogenic processes. In normal regenerative processes involving transient fibrosis FAP-α⁺(myo)fibroblasts disappear from injured tissues, replaced by cells with a normal FAP-α^- phenotype. In chronic uncontrolled pathological fibrosis FAP-α⁺(myo)fibroblasts permanently replace normal tissues. The mechanisms of regulation and elimination of FAP-α expression in(myo)fibroblasts are unknown. According to a yeast two-hybrid screen and protein databanks search, we propose that the intracellular (co)-chaperone BAG6/BAT3 can interact with FAP-α, mediated by the BAG6/BAT3 Pro-rich domain, inducing proteosomal degradation of FAP-α protein under tissue homeostasis. In this Perspective, we discuss our findings in the context of current knowledge on the regulation of FAP-α expression and comment potential therapeutic strategies for uncontrolled fibrosis, including small molecule degraders (PROTACs)-modified FAP-α targeted inhibitors.

The role of Bag3 in cell signaling

Bag3 has been implicated in a wide variety of physiological processes from autophagy to aggresome formation and from cell transformation to survival. We argue that involvement of Bag3 in many of these processes is due to its distinct function in cell signaling. The structure of Bag3 suggests that it can serve as a scaffold that links molecular chaperones Hsp70 and small Hsps with components of a variety of signaling pathways. Major protein-protein interaction motifs of Bag3 that recruit components of signaling pathways are WW domain and PXXP motif that interacts with SH3-domain proteins. Furthermore, Hsp70-Bag3 appears to be a sensor of abnormal polypeptides during the proteotoxic stress. It also serves as a sensor of a mechanical force during mechanotransduction. Common feature of these and probably certain other sensory mechanisms is that they represent responses to specific kinds of abnormal proteins, i.e. unfolded filamin A in case of mechanotransduction or stalled translating polypeptides in case of sensing proteasome inhibition. Overall Hsp70-Bag3 module represents a novel signaling node that responds to multiple stimuli and controls multiple physiological processes.

The Diverse Functions of Small Heat Shock Proteins in the Proteostasis Network

The protein quality control (PQC) system maintains protein homeostasis by counteracting the accumulation of misfolded protein conformers. Substrate degradation and refolding activities executed by ATP-dependent proteases and chaperones constitute major strategies of the proteostasis network. Small heat shock proteins represent ATP-independent chaperones that bind to misfolded proteins, preventing their uncontrolled aggregation. sHsps share the conserved α-crystallin domain (ACD) and gain functional specificity through variable and largely disordered N- and C-terminal extensions (NTE, CTE). They form large, polydisperse oligomers through multiple, weak interactions between NTE/CTEs and ACD dimers. Sequence variations of sHsps and the large variability of sHsp oligomers enable sHsps to fulfill diverse tasks in the PQC network. sHsp oligomers represent inactive yet dynamic resting states that are rapidly deoligomerized and activated upon stress conditions, releasing substrate binding sites in NTEs and ACDs Bound substrates are usually isolated in large sHsp/substrate complexes. This sequestration activity of sHsps represents a third strategy of the proteostasis network. Substrate sequestration reduces the burden for other PQC components during immediate and persistent stress conditions. Sequestered substrates can be released and directed towards refolding pathways by ATP-dependent Hsp70/Hsp100 chaperones or sorted for degradation by autophagic pathways. sHsps can also maintain the dynamic state of phase-separated stress granules (SGs), which store mRNA and translation factors, by reducing the accumulation of misfolded proteins inside SGs and preventing unfolding of SG components. This ensures SG disassembly and regain of translational capacity during recovery periods.

Mechanistic Insights into the Role of Molecular Chaperones in Protein Misfolding Diseases: From Molecular Recognition to Amyloid Disassembly

Age-dependent alterations in the proteostasis network are crucial in the progress of prevalent neurodegenerative diseases, such as Alzheimer’s, Parkinson’s, or amyotrophic lateral sclerosis, which are characterized by the presence of insoluble protein deposits in degenerating neurons. Because molecular chaperones deter misfolded protein aggregation, regulate functional phase separation, and even dissolve noxious aggregates, they are considered major sentinels impeding the molecular processes that lead to cell damage in the course of these diseases. Indeed, members of the chaperome, such as molecular chaperones and co-chaperones, are increasingly recognized as therapeutic targets for the development of treatments against degenerative proteinopathies. Chaperones must recognize diverse toxic clients of different orders (soluble proteins, biomolecular condensates, organized protein aggregates). It is therefore critical to understand the basis of the selective chaperone recognition to discern the mechanisms of action of chaperones in protein conformational diseases. This review aimed to define the selective interplay between chaperones and toxic client proteins and the basis for the protective role of these interactions. The presence and availability of chaperone recognition motifs in soluble proteins and in insoluble aggregates, both functional and pathogenic, are discussed. Finally, the formation of aberrant (pro-toxic) chaperone complexes will also be disclosed.